Development of high efficiency organic solvent extraction liquids has advanced the use of leaching as a process for recovery of metal values. Leaching of metals from ores is an extremely ancient process. Metal ores are contacted with a leach solution that dissolves metal values, producing a pregnant solution. The metal values were originally removed from solution, precipitating them with other substances, typically by means of a lower nobility metal; e.g., scrap iron for recovering copper by cementation. Another recovery route was direct electrowinning. Also, in some instances evaporation was employed. In all those cases the metal product was impure and low quality, so other processes were required for its refining.
Leaching has achieved greater commercial significance in recent years following development of solvent extraction media that can improve the purity and concentration of metals and regenerate leaching solutions. Such a technique employs an organic liquid immiscible with the leaching solution. The organic liquid includes reagents that combine with metal ions and withdraw metal ions from the leaching solution, replacing the metal, normally with hydrogen ion. The metal ions are then stripped from the organic liquid by another aqueous solution that regenerates the hydrogen ion in the organic liquid. For example, in a copper leaching process, a solvent extraction step removes copper from a pregnant solution and replaces the copper ion with two hydrogen ions. The organic solvent extraction medium is then stripped of copper with a sulfuric acid solution that replaces the hydrogen ions in the solvent extraction medium.
The solvent extraction process involves equilibrium between metal ion and hydrogen ion in the aqueous and organic phases. When the hydrogen concentration is high in the organic phase and low in the pregnant solution, metal ion transfers from the pregnant solution to the organic phase. During stripping the hydrogen concentration in the stripping solution is much higher than in the organic phase and exchange occurs to produce a higher metal concentration in the stripping solution than in the pregnant solution. The extraction and stripping steps can each involve plural stages with countercurrent flow to enhance efficiency. The solvent extraction medium is selected for its specificity for particular metal ions for enhancing purity in the stripping solution. For example, a solvent extraction liquid for copper has a much higher affinity for copper than for iron. This, although some iron can be transferred through the solvent extraction medium, copper concentraction can be greatly increased while iron conentration is very much lower.
The solvent extraction process is sometimes referred to as liquid ion exchange
Following solvent extraction the stripping solution is commonly processed in an electrowinning facility that produces metal as cathodes by electrolysis. For example, electrolytic reduction produces high quality cathode copper and simultaneously regenerates the sulfuric acid solution used in solvent extraction stripping.
Electrowinning is a costly operation which may not be readily usable. The capital investment required for electrowinning is relatively high and energy efficiencies are moderate at best. One difficult aspect of electrowinning is the requirement for large amounts of electric power which is often not readily available at remote locations where copper ore is to be processed. This can mean transportation of ore, solution, or electric power, and any can be costly.
Electrowinning may not be suitable for small ore bodies where leaching is useful since a relatively large, long lifetime electrowinning facility is needed for economic reasons. A small ore body can be depleted before the costs of the electrowinning facility are recouped. Electrowinning also uses large amounts of electricity which is costly as compared with other energy sources.
It is therefore desirable to provide a technique for producing metal that can be readily adapted to remote locations and which can be practiced with low capital investment and competitive operating costs.